Showing papers in "Semiconductor Science and Technology in 2006"

DNA-decorated carbon nanotubes for chemical sensing

Abstract: We demonstrate a versatile class of nanoscale chemical sensors based on single-stranded DNA (ssDNA) for chemical recognition and single-walled carbon nanotube field effect transistors (SWNT FETs) for electronic read-out. SWNT FETs with a nanoscale coating of ssDNA respond to vapours that cause no detectable conductivity change in bare devices. The gases tested are methanol, trimethylamine, propionic acid, dimethylmethylphosphonate and dinitrotoluene. Sensor responses differ in sign and magnitude for different gases and can be tuned by choice of the ssDNA base sequence. Sensors respond and recover rapidly (seconds), and the sensor surface is self-regenerating. Preliminary results of all-atom molecular dynamics simulations are consistent with experiment.

White organic LEDs and their recent advancements

Abstract: White organic light emitting devices (WOLEDs) are being considered as substitutes for conventional white light sources. They are efficient solid-state lighting sources and their power efficiencies have surpassed those of the incandescent light sources, especially due to recent improvement in device architectures, molecular engineering in synthesis of new materials and the incorporation of electrophosphorescent emitters. This paper reviews the various approaches to achieve white light emission from organic light emitting diodes (OLEDs), their advantages, disadvantages and recent progress. The device architecture and problems related to various device designs have been discussed.

Heterojunctions and superlattices based on silicon carbide

Abstract: In addition to possessing unique electrical properties, silicon carbide (SiC) can crystallize in different modifications (polytypes). Having the same chemical nature, SiC polytypes may significantly differ in their electrical parameters. In recent years, the world's interest in the fabrication and study of heteropolytype structures based on silicon carbide has considerably increased. This review considers studies concerned with polytypism in SiC, fabrication technologies of various types of heterostructures constituted by different SiC polytypes and their electrical parameters. It is shown that heterostructures between SiC polytypes may have a better structural perfection than those constituted by semiconductors that differ in chemical nature. A conclusion is made that SiC-based heterostructures are promising for application in modern electronic devices.

Sub-bandgap photoconductivity in ZnO epilayers and extraction of trap density spectra

Abstract: Photoconductivity is observed in ZnO epilayers due to photoexcitation in the visible spectral region of 400–700 nm, below the ZnO bandgap energy of 3.4 eV. Photoconductive transients due to visible photoexcitation have time constants in the order of minutes. Treatment of the ZnO surface with SiO2 passivation layers results in a significant reduction in the photoconductive signal and photoconductive time constant. The photoconductive response is attributed to hole traps in ZnO, where a rate equation model is presented to describe the photoconductive characteristics. A method of extracting the hole trap density spectrum is presented on the basis of the rate equation model and assumptions for hole capture lifetime and carrier recombination lifetime that are validated by experimental time-resolved photoluminescence measurements of the material under study. Traps are found to be distributed near 0.75 eV and 0.9 eV from the valence band edge for SiO2 passivated and unpassivated ZnO epilayers, respectively.

Investigation of a p-CuO/n-ZnO thin film heterojunction for H2 gas-sensor applications

Abstract: A p-CuO/n-ZnO thin film heterojunction is fabricated on a glass substrate by the sol–gel technique. The crystallinity of the junction materials and microstructure of the top p-layer are examined by an x-ray diffractometer (XRD) and scanning electron microscope (SEM). The current–voltage (I–V) characteristics of the p–n heterojunction and its temperature dependence have been investigated in air and H2 ambient. Although the junction possesses linear I–V characteristics from room temperature (RT) to 150 °C in air, at higher temperatures (200 °C to 300 °C), it shows nonlinear rectifying behaviour. The forward current is greatly increased with increasing temperature while the reverse current is increased slightly resulting in a IF/IR ratio as high as 485. The ideality factor (n) is 4.88 at a temperature of 300 °C. The forward current is highly increased by the introduction of H2 gas at 300 °C. However, a simultaneous increase in the reverse current makes the IF/IR ratio 8.4. It is observed that H2 sensitivity of the heterojunction is increased with the increase in temperature as well as the thickness of CuO film. A sensitivity value as high as 266.5 is observed at 300 °C when biased at 3 V in the presence of approximately 3000 ppm of H2.

Nanospintronics with carbon nanotubes

Abstract: One of the actual challenges of spintronics is the realization of a spin transistor allowing control of spin transport through an electrostatic gate. In this paper, we report on different experiments which demonstrate gate control of spin transport in a carbon nanotube connected to ferromagnetic leads. We also discuss some theoretical approaches which can be used to analyse spin transport in these systems. We emphasize the roles of the gate-tunable quasi-bound states inside the nanotube and the coherent spin-dependent scattering at the interfaces between the nanotube and its ferromagnetic contacts.

Atomistic calculation of elastic moduli in strained silicon

Abstract: Strained silicon is becoming a new technology in silicon industry where the novel strain-induced features are utilized. In this paper we present a molecular dynamic prediction for the elastic stiffnesses C11, C12 and C44 in strained silicon as functions of the volumetric strain level. Our approach combines basic continuum mechanics with the classical molecular dynamic approach, supplemented with the Stillinger–Weber potential. Using our approach, the bulk modulus, effective elastic stiffnesses C11, C12 and C44 of the strained silicon, including also the effective Young's modulus and Poisson's ratio, are all calculated and presented in terms of figures and formulae. In general, our simulation indicates that the bulk moduli, C11 and C12, increase with increasing volumetric strain whilst C44 is almost independent of the volumetric strain. The difference between strained moduli and those at zero strain can be very large, and therefore use of standard free-strained moduli should be cautious.

Influence of annealing on physical properties of evaporated SnS films

Abstract: The effect of annealing on the composition, crystal structure, surface features and electro-optical properties of tin mono-sulfide (SnS) films, deposited by thermal evaporation at $300^\circ C$, has been studied. Elemental analysis of the films shows sulfur deficiency, which increases at higher annealing temperatures $(T_a)$. The SnS structure in the as-deposited and annealed films remains orthorhombic. With an increase in $Ta_$, the grain size and the surface roughness are reduced. The electrical resistivity also decreases with increasing $T_a$. The variation of activation energy and optical parameters with $Ta_$ has been explained by taking into account the degree of preferred orientation of the grains. The films annealed at $100^\circ C$ show some unusual features compared to those annealed at other temperatures.

The effect of substrate temperature on the properties of ITO thin films for OLED applications

Abstract: Indium tin oxide (ITO) thin films have been deposited by rf magnetron sputtering on glass substrates at different substrate temperatures. The structural, electrical and optical properties of these films have been investigated to obtain optimum values for resistivity, optical transmittance and surface smoothness. The film deposited at a substrate temperature of 300 °C shows good conductivity, optical transmittance, crystallinity and surface smoothness. These ITO films were used to fabricate organic light emitting diodes (OLEDs). The dc current–voltage (I–V) studies on ITO/PEDOT:PSS/MEH-PPV/Al test structures show better rectifying behaviour on a smoother ITO substrate.

The double Gaussian distribution of barrier heights in Au/n-GaAs Schottky diodes from I–V–T characteristics

Abstract: The current?voltage (I?V) and capacitance?voltage (C?V) characteristics of Au/n-GaAs contacts have been measured in the temperature range of 80?300 K. An abnormal decrease in the experimental BH ?b and an increase in the ideality factor n with a decrease in temperature have been observed. This behaviour has been attributed to the barrier inhomogeneities by assuming a Gaussian distribution of barrier heights at the metal?semiconductor interface. The temperature-dependent I?V characteristics of the Au/n-GaAs contact have shown a double Gaussian distribution giving mean barrier heights of 0.967 and 0.710 eV and standard deviations of 0.105 and 0.071 V, respectively. A modified ln(I0/T2) ? q2?2s/2k2T2 versus 1/T plot for the two temperature regions then gives and A* as 0.976 and 0.703 eV, and 13.376 and 8.110 A cm?2 K?2, respectively. Furthermore, a value of ?0.674 meV K?1 for the temperature coefficient has been obtained, and the value of ?0.674 meV K?1 for the Au/n-GaAs Schottky diode is in close agreement with those in the literature.

Performance assessment of nanoscale double- and triple-gate FinFETs

Abstract: Based on 3D simulations, we report a performance assessment of triple- and double-gate FinFETs for high performance (HP), low operating power (LOP) and low standby power (LSTP) logic technologies according to ITRS 65 nm node specifications. The impact of spacer width, lateral source/drain doping gradient, aspect ratio, fin thickness and height along with gate work function on the device performance has been analysed in detail and guidelines are presented to meet the ITRS projections. The design guidelines proposed for a 65 nm node are also examined for a 45 nm node for triple- and double-gate FinFETs. Results show that lateral source/drain doping gradient along with spacer width can not only effectively control short channel effects, thus presenting low off-current, but can also be optimized to achieve low values of intrinsic delay. FinFETs should be designed with a higher aspect ratio (~4) along with lower values of fin thickness to achieve ITRS targets for off-current and intrinsic delay. Triple-gate FinFETs show greater design flexibility in selecting important technological and device parameters as compared to double-gate devices. A design window is presented to achieve ITRS targets for the three logic technology requirements with triple- and double-gate FinFETs.

Diffusion of silicon in crystalline germanium

Abstract: We report the determination of the diffusion coefficient of Si in crystalline Ge over the temperature range of 550 to 900 C. A molecular beam epitaxy (MBE) grown buried Si layer in an epitaxial Ge layer on a crystalline Ge substrate was used as the source for the diffusion experiments. For samples annealed at temperatures above 700 C, a 50 nm thick SiO{sub 2} cap layer was deposited to prevent decomposition of the Ge surface. We found the temperature dependence of the diffusion coefficient to be described by a single activation energy (3.32 eV) and pre-factor (38 cm{sup 2}/s) over the entire temperature range studied. The diffusion of the isovalent Si in Ge is slower than Ge self-diffusion over the full temperature range and reveals an activation enthalpy which is higher than that of self-diffusion. This points to a reduced interaction potential between the Si atom and the native defect mediating the diffusion process. For Si, which is smaller in size than the Ge self-atom, a reduced interaction is expected for a Si-vacancy (Si-V{sub Ge}) pair. Therefore we conclude that Si diffuses in Ge via the vacancy mechanism.

On the intersecting behaviour of experimental forward bias current–voltage (I–V) characteristics of Al/SiO2/p-Si (MIS) Schottky diodes at low temperatures

Abstract: In this study, we have investigated the intersection behaviour of forward and reverse bias current–voltage (I–V) characteristics of Al/SiO2/p-Si Schottky diodes in the temperature range of 79–325 K. The crossing of the experimental semi-logarithmic ln(I)–V curves appears as an abnormality when seen with respect to the conventional behaviour of ideal Schottky diodes. Experimental results show that this crossing of ln(I)–V curves is an inherent property of even Schottky diodes. The ideality factor n was found to decrease, while the zero-bias Schottky barrier height (SBH) ΦB0 increases with increasing temperature. The conventional Richardson plot is found to be nonlinear in the temperature range measured. However, the ln(I0/T2) versus 1000/nT plot gives a straight line corresponding to activation energy 0.233 eV. It is shown that the values of series resistance Rs estimated from Cheung's method were strongly temperature dependent and abnormally increased with increasing temperature. In addition, the temperature dependence of energy distribution of interface states density Nss profiles was obtained from the forward bias I–V measurements by taking into account the bias dependence of the effective barrier height Φe and ideality factor n. All these behaviours indicate that the thermionic emission (TE) cannot be the main current transport mechanism, especially at low temperatures.

Complete UV emission of ZnO nanoparticles in a PMMA matrix

Abstract: Preparation and photoluminescence (PL) properties of zinc oxide (ZnO) nanoparticles embedded in a lipophilic polymethyl methacrylate (PMMA) matrix are reported in detail with an unbalanced sol–gel route. A high-resolution transmission electron microscope (HRTEM) indicates that ZnO particles are highly crystallized, with a size of 5–6 nm and hexagonal wurtzite structure. During the sol–gel reaction, partial ester groups of R-COOCH3 in PMMA are hydrolyzed to form carboxylic ion groups, which chemisorb on the surface of ZnO nanoparticles to eliminate the defects; thus ZnO nanoparticles in the PMMA matrix exhibit complete ultraviolet (UV) emissions, while emissions in the visible region are fully quenched.

Grain and crystal texture properties of absorber layers in MOCVD-grown CdTe/CdS solar cells

Abstract: The microstructure of 4?13 ?m thick CdTe absorber layers in CdTe/CdS/ITO/glass solar cell structures grown by metal-organic chemical vapour deposition (MOCVD) at 350 ?C has been studied. The crystalline texture, lattice parameter and grain size were measured as a function of thickness for the as-grown layers, and as a function of annealing temperature and time for annealing in both nitrogen (N2) and cadmium chloride (CdCl2) environments. The average grain sizes developed with thickness as r (?m) = 0.050x ? 0.10 (4 < x < 12 ?m), and this behaviour is contrasted with that for close-spaced sublimation material grown at 500 ?C. Annealing in both ambients promoted grain growth (with Rayleigh grain size distribution functions and Burke?Turnbull exponents being n = 7 at 440 ?C and ~4 at 400 ?C), a development of the grown-in preferred orientation from [1?1?1] to [2?1?1], and relief of the grown-in compressive stress. A growth mechanism by which development of the [2?1?1] preferred orientation may accompany grain growth is described. It is concluded that MOCVD growth at temperatures higher than 350 ?C used here will be required to produce the larger grain sizes required for photovoltaic applications.

Multi sub-band Monte Carlo simulation of an ultra-thin double gate MOSFET with 2D electron gas

Abstract: A new two-dimensional self-consistent Monte Carlo simulator including multi sub-band transport in 2D electron gas is described and applied to thin-film SOI double gate MOSFETs. This approach takes into account both out of equilibrium transport and quantization effects. Our method allows us to significantly improve microscopic insight into the operation of deep sub-100 nm CMOS devices. We compare and analyse the results obtained with and without quantization effects for a 15 nm long DGMOS transistor.

The effect of Schottky metal thickness on barrier height inhomogeneity in identically prepared Au/n-GaAs Schottky diodes

Abstract: We have studied identically prepared Au(5 nm)/n-GaAs (35 dots) and Au(65 nm)/n-GaAs (38 dots) Schottky barrier diodes (SBDs) on the same n-type GaAs single crystal. A GaAs wafer has been prepared by the usual chemical etching, and evaporation of the metal has been carried out in a conventional vacuum system. The effective Schottky barrier heights (SBHs) and ideality factors obtained from the current–voltage (I–V) characteristics have differed from diode to diode. The SBH for the Au(5 nm)/n-GaAs diodes have ranged from 0.839 to 0.943 eV and the ideality factor n from 1.011 to 1.150. The SBH for the Au(65 nm)/n-GaAs diodes have ranged from 0.828 to 0.848 eV and the ideality factor n from 1.026 to 1.069. Our aim is to find the laterally homogeneous SBH values of the SBDs depending on Schottky metal thickness. The lateral homogeneous SBH values of 0.940 eV for the Au(5 nm)/n-GaAs and 0.866 eV for the Au(65 nm)/n-GaAs diodes have been calculated from a linear relationship between barrier height (BH) and the ideality factor, which can be explained by lateral inhomogeneities of the SBH, respectively.

An investigation of annealing on the dielectric performance of TiO2 thin films

Abstract: The impact of annealing on the dielectric performance of TiO2 thin films synthesized by PECVD was investigated. Films annealed between 500 and 700 °C have an anatase crystal structure, while 800 °C annealed films display the rutile phase. The optimal annealing temperature was 600 °C, which both maximized the dielectric constant and minimized the leakage current density. The intrinsic dielectric constant of TiO2 improved from 82 ± 10 in as-deposited films to 168 ± 30 after annealing. The leakage current of optimized films was superior to the SiO2 control samples over a range of equivalent oxide thickness. Fowler–Nordheim tunnelling and Frenkel–Poole conduction were observed in the optimized films, while Schottky emission dominated leakage current at other conditions.

A high-quality SOI structure fabricated by low-temperature technology with B+/H+ co-implantation and plasma bonding

Abstract: In order to decrease the heating temperature of Smart-Cut technology, plasma activation and B+/H+ co-implantation were used. B+/H+ co-implantation gave rise to a decrease of splitting temperature, and finally a high-quality silicon-on-insulator (SOI) structure was fabricated at 300 °C. A diluted Secco etching test indicated that the top Si surface defect density was 104–105 cm−2. An atomic force microscope (AFM) image showed that the surface roughness of the top Si layer was much smaller than that of the SOI structure formed from the H-only implanted wafer. The diluted Secco etching test also indicated that the top Si surface defect density would decrease with the increase of annealing temperature, and 500 °C was a critical temperature for reducing the defect density.

Ultrananocrystalline diamond for electronic applications

Abstract: Ultrananocrystalline diamond is a unique form of carbon with grain sizes in the 3–5 nm region. This nanostructure has profound implications on electronic transport, as ~10% of carbon is at the grain boundaries. Thus, this material has significant π bonding which governs the majority of the electrical conductivity due to the lower energy gap of π–π* transitions relative to σ–σ* transitions. The addition of nitrogen into the gas phase during deposition promotes n-type conductivity, due to the increase in the density of states associated with π bonding. This material is not doped in the conventional sense, and its applications lie in the electrode/metallic conductivity region rather than in the more moderately doped active device regime. This review paper aims to describe the origin and behaviour of the conductivity mechanism, as well as briefly review some applications.

Quantum dots in carbon nanotubes

Abstract: In this overview paper, we present low-temperature electronic transport measurements of carbon nanotube quantum dots with a back gate. In a semiconducting tube, charge carriers could be completely depleted. The addition energy and the excitation spectrum have been studied as a function of the number of charges (electrons or holes), one by one. We observe electron–hole symmetry, which is a direct consequence of the symmetric band structure of the nanotube. The excitation spectrum for metallic nanotubes exhibits four-fold shell filling and is completely described by an extended constant-interaction model. Furthermore, nanotubes with a four-fold shell structure are investigated in a parallel magnetic field. The magnetic field induces a large splitting between the two orbital states of each shell, demonstrating their opposite magnetic moment and determining transitions in the spin and orbital configuration of the quantum dot ground state. Also, a small coupling is found between orbitals with opposite magnetic moments leading to anti-crossing behaviour at zero field. Current–voltage characteristics of suspended carbon nanotube quantum dots show an additional series of steps equally spaced in voltage. The energy scale of this harmonic, low-energy excitation spectrum is consistent with that of the longitudinal low-k phonon mode (stretching mode) in the nanotube. Finally, we report on a fully tunable carbon nanotube double quantum dot. We perform inelastic transport spectroscopy via the excited states in the double quantum dot, a necessary step towards the implementation of new microwave-based experiments for quantum information technology.

Characterization of Ge-on-Si virtual substrates and single junction GaAs solar cells

Abstract: A virtual substrate consisting of a Ge layer grown directly on Si without an intervening SiGe graded layer is characterized. The nominally 100% Ge overlayer is fully relaxed and contains a small amount (3%) of unintentional Si. A dislocation density of 108 cm−2 is estimated for the virtual substrate prior to GaAs epitaxial growth, which is reduced by a factor of 100 after the growth of GaAs. On this novel virtual substrate 1 cm2 single-junction GaAs photovoltaic cells were realized with an efficiency of 11.7% under AM0 compared with 20.2% for cells grown on a crystalline Ge substrate. Due to the high dislocation density a 50-fold higher dark current is measured in the virtual substrate cells compared to the crystalline Ge cells, leading to a lower short circuit current and open-circuit voltage of the cells fabricated on the virtual substrates. The post-GaAs growth dislocation density is estimated as 1 × 107 cm−2 in the base region and 4 × 105 cm−2 in the emitter region based on modelling and measurements.

Photoluminescent properties of InN epifilms

Abstract: We report a detailed investigation of the photoluminescent properties of InN epifilms with free-electron concentrations ranging from 3.5 × 1017 cm−3 to 5 × 1019 cm−3. It is found that the photoluminescence (PL) peak energy strongly depends on the electron concentration. We show that the broadening of the PL spectra with increasing free-electron concentration arises from the breaking of the k = 0 selection rule. The large asymmetric line shape of the photoluminescence spectra can be well described by the free-electron recombination band model. We establish an empirical relation between the full-width at half-maximum (FWHM) value of the PL spectra and the free-electron concentration, which provides a convenient formula to determine the free-electron concentration in InN epifilms by PL measurement. We point out that the peak energy of the PL spectra does not reflect the real band gap of InN epifilms. Calculations based on the effects of Burstein–Moss absorption, band tail and band renormalization were used to analyse the PL spectra, and the fundamental band gap of the intrinsic InN film was obtained. The corresponding expression for the band gap narrowing effect of the InN film is found to be ΔEBGN = 1 × 10−8n1/3 + 3.6 × 10−7n1/4 + 2.3 × 10−11n1/2 eV. The temperature-dependent band gap of the intrinsic InN was fitted by the Passler equation. The Passler parameters of the intrinsic InN are α = 0.55 meV K−1, Θ = 576 K and p = 2.2. It is found that the band gap energies at T = 0 K and room temperature are close to 0.68 eV and 0.62 eV, respectively. In addition, we show that the band gap obtained from the PL spectra is in excellent agreement with that obtained from infrared absorption.

Hydrogenated aluminium-doped zinc oxide semiconductor thin films for polymeric light-emitting diodes

Abstract: Highly conducting transparent thin films of aluminium(Al)-doped zinc oxide (ZnO:Al) were deposited by a radio frequency magnetron-sputtering technique using an argon and hydrogen gas mixture at room temperature. Hydrogen serves as a shallow donor and plays a critical role in improving the Al doping efficiency to enhance the conductivity of thin films. The effect of hydrogen partial pressure on the properties of ZnO:Al films was investigated in detail. Polycrystalline ZnO:Al films with a surface roughness of about 2 nm, conductivity of 1.97 ? 103 S cm?1, transmittance of over 83% in the visible wavelength region and an optical band gap of 3.93 eV were achieved at a hydrogen partial pressure of 7.5 ? 10?4 Pa. A ZnO:Al film with the desired properties was used as an anode contact in a bi-layer polymeric light-emitting diode. A polyethylene dioxythiophene?polystyrene sulfonate doped with poly(styrenesulfonic acid) (PEDOT: PSS) and phenyl-substituted poly(p-phenylene vinylene) (Ph-PPV) were employed as a hole transport layer and a light-emitting layer, respectively. The electro-luminescence performance of the aforementioned diode was studied and compared to a control device with an indium tin oxide anode.

Capacitance characteristics of MOS capacitors embedded with colloidally synthesized gold nanoparticles

Abstract: In this work, the capacitance characteristics of gold nanoparticle-embedded metal–oxide-semiconductor (MOS) capacitors with Al2O3 control oxide layers are investigated The capacitance versus voltage (C–V) curves obtained for a representative MOS capacitor embedded with gold nanoparticles synthesized by the colloidal method exhibit large flat-band voltage shifts, which indicate the presence of charge storages in the gold nanoparticles Their hysteresis characteristics are dependent on the voltage sweep range The clockwise hysteresis and rightward shift of the flat band voltages observed from the C–V curves imply that electrons are trapped in a floating gate layer consisting of the gold nanoparticles present between SiO2 and Al2O3 layers in the MOS capacitor, and that these trapped electrons originate from the top electrode In addition, the characteristics of the capacitance versus time curves for the gold nanoparticle-embedded MOS capacitor are discussed in this paper

CuInS2 thin films grown sequentially from binary sulfides as compared to layers evaporated directly from the elements

Abstract: CuInS2 thin films have been obtained by rapid thermal annealing at 350 °C of binary sulfide precursors evaporated at a substrate temperature lower than 200 °C. The structure, morphology, and chemical and optical properties of these samples have been analysed and compared to CuInS2 layers prepared directly by evaporation from the elements onto 350 °C heated substrates. X-ray diffraction and x-ray photoelectron spectroscopy measurements showed similar structure and chemical characteristics for the CuInS2 samples prepared by sequential and direct routes. Some differences in the optical absorption can be related to morphological variations observed by atomic force microscopy.

Performance of concentrator solar cells with passive cooling

Abstract: This paper presents the results of an experimental study regarding the role of passive cooling on silicon-based concentrator solar cell performance. A design is presented for a gravity-dependent copper heat pipe using water or acetone as the working fluid for passively cooling concentrator solar cells operating up to 500×. Heating of the solar cell by illumination and temperature-dependent properties is taken into account. In addition, the performance at high concentrations has been improved using acetone as the working fluid in the gravity-dependent copper heat pipe for passive cooling. The results are promising for reasonable costs for such high efficiency cells working up to 500 suns.

A comparative study of BSF layers for GaAs-based single-junction or multijunction concentrator solar cells

Abstract: An effective back surface field is a key structural element for a high-efficiency GaAs concentrator solar cell, either in a multijunction or in a single-junction device. In this paper, several BSF materials are analysed, namely: (1) p++GaAs(Zn), (2) p+Ga0.5In0.5P(Zn) and (3) p++Al0.2Ga0.8As(C). The results of the comparison demonstrate that the best option is C-doped Al0.2Ga0.8As, which exhibits a low series resistance and behaves as an excellent minority carrier mirror; p++GaAs(Zn) shows reduced minority carrier mirror properties resulting from Zn diffusion and p+Ga0.5In0.5P(Zn) is shown to produce important series resistance problems because of an unfavourable heterojunction with GaAs.

Gate control of the giant Rashba effect in HgTe quantum wells

Abstract: HgTe/Hg0.3Cd0.7Te(0 0 1) quantum well structures fabricated with a Si–O–N insulator layer and an Au top gate electrode exhibit hysteresis effects in their gate-voltage dependent carrier density and thus a nonlinear variation of the Rashba spin–orbit splitting energy (ΔR). Charging and discharging of states at the semiconductor insulator interface has been found to be responsible for this effect. The quantitative agreement with a simple capacitor model has been used to identify the maximum hysteresis-free gate-voltage range. A nearly linear variation of ΔR with applied gate voltage has been observed in this range.

The effect of substrate surface on the physical properties of SnS films

Abstract: The effect of substrates on the physical properties of tin mono-sulphide (SnS) films has been studied. The SnS films were deposited using the resistive thermal evaporation method on CORNING 7059 glass, ITO-coated glass, Si wafer and Ag-coated glass substrates. The as-deposited films exhibited nearly stoichiometry between Sn and S elements with a Sn/S at.\% ratio of ~1.05. Structural analysis of these films indicated that the films are crystallized in the form of an orthorhombic crystalline structure and showed (1 1 1) as a dominant peak, except for the films grown on Si substrates. Si/SnS films exhibited (0 4 0) as a dominant peak. The ITO/SnS films showed high values of rms roughness (~14.9 nm) and average grain size (~225 nm), along with a low electrical resistivity of $8.9 x 10^{-3} \omega cm$ as compared to SnS films grown on glass, Si and Ag substrates. The ITO/SnS films exhibit low resistivity, probably due to the large size of grains, and could be suitable for optoelectronic device applications.